It is generally known to transform harmful components of exhaust gases of an internal combustion engine such as HC, NO.sub.x and CO into substantially non-toxic gases by means of a catalyzer which is mounted in the exhaust-gas system of an internal combustion engine.
However, it is decisive for the so-called conversion rate that the oxygen content of the exhaust gas lies within optimal values. For a so-called three-way catalyzer, these optimal values lie in a narrow range about the value which corresponds to an air/fuel mixture of lambda=1.
In order to maintain this narrow range, it is conventional, as is well known, to control the air/fuel ratio for an internal combustion engine by means of oxygen probes (lambda probes) which are disposed in the exhaust gas system of the internal combustion engine.
In addition to the control on the basis of the signal of the oxygen probe, the determination of a so-called precontrol value based on operating characteristic variables of the internal combustion engine especially the air quantity Q and the engine speed n takes place in order to accelerate the control operation especially in the crossover regions. The determination of the air quantity Q can take place in various ways such as via the determination of the opening angle of a throttle flap or based upon the signal of an air-flow sensor.
The precontrol value determined on the basis of air quantity and engine speed is corrected in dependence upon the signal of the oxygen probe in such a manner that the optimal air/fuel mixture is determined. This corrected signal then drives a fuel-metering device which supplies the optimal quantity of fuel to the internal combustion engine.
If a fuel injection unit is used as the fuel-metering device, then the drive signal supplied to the injection device defines a so-called injection time ti which defines a direct measure for the quantity of fuel supplied per work stroke for the necessary conditions such as constant fuel pressure ahead of the injection valves and the like.
For other fuel-metering devices, their drive signal is to be correspondingly determined. This is known to persons working in the field. In the following, the invention will be described in the context of a fuel injection device without being limited thereto.
Furthermore, in U.S. patent application Ser. No. 679,050, filed May 9, 1991, a system has been introduced which utilizes two lambda probes for controlling the air/fuel mixture with a first lambda probe being disposed forward of a catalyzer and the second lambda probe being disposed rearward thereof.
The signal of the second lambda probe is compared to a desired value with the difference of the two values being integrated and the value obtained in this way serves as the desired value for the signal of the first lambda probe.
A system is further known from U.S. Pat. No. 4,622,809 which utilizes the signal of a lambda probe arranged ahead of the catalyzer for controlling the air/fuel mixture and the signal of a second lambda probe for monitoring the conversion rate of the catalyzer.
The signal value supplied by the second probe and its mean value are determined, and the operating point of the control system is changed as long as there is a deviation of the mean value from a pregiven value until the mean value has reached its desired value.
If the mean value is at the pregiven value, then a fault signal is supplied when maximum values of the amplitude of the sensor signal are exceeded. In this way, the necessity is signalled when required to exchange the catalyzer.